Imaging spectrometers are, broadly speaking, optical instruments which process the electromagnetic radiation from a source into its fundamental components. For instance, an interferometric based spectrometer divides light from a source and interferes it to produce a fringe pattern of interfering light (i.e., an interferogram). The interference pattern can be captured on film or by, for instance, an electronic detector, for example, a semi-conductor array detector (e.g., a charged coupled device (CCD)).
There are numerous optical interferometer designs. The basic form of the Sagnac (or common path) interferometer is illustrated in FIG. 1. It is also illustrated in U.S. Pat. No. 4,976,542 to Smith. Other designs include the Mach-Zender interferometer, the Michelson interferometer and Twyman-Green interferometer (See W. L. Wolfe, Introduction to Imaging Spectrometers, SPIE Optical Engineering Press, pp. 60–64, 1997), the Fabry-Perot interferometer (see Wolfe, p. 70–73), the Lloyd's mirror interferometer (see the Smith patent) and, a variation of the common path interferometer (Sagnac) sometimes referred to as the Barnes interferometer (see T. S. Turner Jr., et al., A Ruggedized Portable Fourier Transform Spectrometer for Hyperspectral Imaging Applications, SPIE Vol. 2585 pp 222–232.) There are also dispersive spectrometers such as prism spectrometers and grating spectrometers. (See Wolfe, pp. 50–52 and 55–57).
In a non-imaging Fourier transform spectrometer the point source of radiation is split into two virtual points a fixed distance apart to yield a fringe pattern at the detector. If one wants to attain a fine spectral resolution, the distance between the two virtual points should be large; for a course spectral resolution, it should be short. This distance may be controlled by shifting one of the mirrors (typically referred to as lateral shear) of, for instance, the common path interferometer. With this arrangement, a wide spectral range measurement loses resolution, while a high resolution measurement reduces the effective spectral range. In an imaging spectrometer, the point source is imaged with a set of imaging optics and a slit is inserted giving the instrument the capability of one-dimensional imaging in the direction perpendicular to the shear.
Shear, both lateral and angular, is discussed in Turner, Jr. et al. (supra). For the Sagnac, translation of either mirror in the plane of FIG. 1 produces lateral shear. Mirror tilt about an axis perpendicular to the drawing plane also produces lateral shear. Conversely, in the Barnes interferometer only angular shear is possible and is produced only by mirror tilt. See FIGS. 2 and 3 of Turner, Jr., et al.
U.S. Pat. No. 4,976,542 to W. H. Smith discloses a Fourier transform spectrometer which incorporates the common path (or Sagnac) interferometer and in which a charge-coupled device (CCD) is placed in the image plane instead of film. The CCD has pixels aligned along two dimensions to provide both spectral resolution and spatial resolution. The CCD is characterized by greater dynamic range, lower pixel response variation, and is photon nose limited, all of which enhances its use as a detector for a spectrometer. See also Digital Array Scanned Interferometers for Astronomy, W. H. Smith, et al., Experimental Astronomy 1: 389–405, 1991. In these devices, the interferometer introduces lateral shear in one direction and a two dimensional camera is aligned so a row of pixels is parallel to this geometric plane. In the perpendicular direction, a set of cylindrical lenses is used to provide an imaging capability along the columns of pixels. A row plot from the detector is an interferogram similar to the interferogram collected in a temporally modulated Michelson interferometer.
In a paper published in 1985, T. Okamoto et al. describe a method for optically improving the resolving power of the photodiode array of a Fourier transform spectrometer by modulating the spatial frequency of the interferogram with a dispersing element. With the use of a dispersing element, particularly an optical element with parallel surfaces, the distance between the two virtual sources varies with the wave number (the inverse of wavelength) of the source. Thus, as illustrated in FIG. 2 of this reference, by placing their optical dispersive element into the optical path of a common path interferometer, the distance between the virtual source becomes a function of the wave number (i.e., the optical dispersive element refracts the blue beam more than the red beam, yielding a wide distance between S1blue and S2blue and a narrower distance between S1red and S2red). The authors claim that use of the optical parallel greatly enhances the resolution. In principle, the spectrometer can be designed to examine any wavelength band of interest by careful choice of the type of dispersive glass utilized and the thickness of the glass. See “Optical Method for Resolution Enhancement in Photodiode Array Fourier Transform Spectroscopy,” T. Okamoto et al, Applied Optics Vol. 24, No. 23, pp 4221–4225, 1 Dec. 1985.
The approach of Okamoto et al. has a number of drawbacks. First, because of the use of the dispersive block, the system no longer operates with constant wave number increments. This is in contrast with conventional Fourier transform spectrometers, which are constant wave number devices and are inherently spectrally calibrated. Thus, with Okamoto et al., blue wavelengths have a much smaller spectral resolution than red wavelengths, and the spectral calibration of the instrument becomes a major issue. Another drawback is that the spectral dispersion, while it enhances spectral resolution, adversely affects spatial resolution. Thus, the dispersive element would greatly increase the complexity of an imaging Okamoto et al. spectrometer. Another disadvantage of this technique is that its dependence on a dispersive material restricts its use to wavelengths that can be effectively transmitted through a dispersive element. Finally, the limited glass types that are available restrict the range of spectral enhancements available. While it is theoretically possible to use any dispersive glass and increase the size of the block to achieve the desired spectral enhancement, in practice the size of the block may become so large that the instrument is no longer practical. Also, since the enhancement depends on the glass type and size, the instrument designer has a limited number of parameters to use to optimize the spectrometer design and may not be able to arbitrarily set the lower and upper limits of the spectral region of interest.
In “Spatial Heterdoyne Spectroscopy: A Novel Interferometric Technique for the FUV,” J. Harlander et al., SPIE Vol. 1344, pp. 120–131 (1990), the authors describe an improved interference spectrometer which has no moving parts, can be field widened, and can be built in an all reflection configuration for UV applications, particularly FUV applications. Harlander et al. are addressing a different problem from that addressed in Okamoto et al. and approach their solution in a different manner (e.g., the use of angular shear instead of the lateral shear required by Okamato et al.). The basic concept (illustrated in FIG. 1 of this reference) is based on a Michelson type interferometer in which the return mirrors are replaced by diffractive gratings. These gratings, which disperse the radiation, produce Fizeau fringes (i.e., interferograms) which are recorded by a detector positioned in the image plane. The Fourier transform of the fringe pattern recovers the spectrum. An all reflection version of the foregoing utilizes a collimator, a diffraction grating and two mirrors. Light from the collimator is split into two beams by the first half of the diffraction grating, which travel in different directions until they are recombined by the second half of the same grating and focused onto the detector by a mirror. This is illustrated in FIG. 2 of this reference. See also, “Spatial Hetrodyne Spectroscopy for the Exploration of Diffuse Interstellar Emission Lines at For-Ultraviolet Wavelengths,” J. Harlander et al., The Astrophysical Journal, 396: 730–740, 1992 Sep. 10, and U.S. Pat. No. 5,059,027 to Roesler et al. All the designs suggested/disclosed require the use of collimated light and angular shear.
There are a number of drawbacks/limitations associated with the designs suggested/disclosed in the above referenced Harlander et al. publications and Roesler, et al. patent (collectively “Harlander et al.”). First of all, Harlander et al. do not disclose the concept of imaging a spatially varying scene. Their invention is discussed in the context of imaging a star or some other type of point source. They implicitly assume that the light coming into their optical system is homogenous and report a single spectra. In many cases this may not be true, and proper measurement of the scene would require spectra for each spatial element in the scene. Secondly, all of the Harlander et al. designs require collimating the input beam. Such designs are inherently more complicated than designs which do not require collimated light. Third, the Michelson design on which their designs are based is inherently less mechanically stable than the common path design, since the interferometer is not self-compensating for motions in the elements of the interferometer. It is also not clear if the concept of Harlander et al. is applicable to instruments which utilize lateral shear, as opposed to angular shear. Fourth, although not explicitly stated, all the designs of Harlander et al. require a re-imaging lens to image the virtual sources at infinity. Finally, Harlander et al. require a complex method for separating wavelengths below the central wavelength from those above the central wavelength. That is, a detected fringe pattern could have two different interpretations, it could be from a source a below the central wavelength or Δλ above. Harlander, et al., discusses methods for determining the true wavelength.